The production of acyclic olefinic hydrocarbons by dehydrogenation is a highly useful hydrocarbon conversion process. The product olefinic hydrocarbons find utility in the production of a wide variety of useful chemicals including synthetic lubricants, detergents, polymers, alcohols, plasticizers, etc.
Modern catalytic dehydrogenation processes provide a high degree of selectivity to the formation of linear monoolefins. However, they are still troubled by the production of by-products, basically due to thermal cracking reactions and to undesired catalytic dehydrogenation side reactions. The by-products fall into three broad classes, light hydrocarbons formed by cracking reactions, diolefinic hydrocarbons having the same carbon number as the desired monoolefinic hydrocarbons, and aromatic hydrocarbons also having the same carbon number as the desired monoolefinic hydrocarbons. The production of aromatic hydrocarbons is most troublesome, especially when the objective is to produce high purity monoolefinic hydrocarbons. The light hydrocarbons which result from cracking reactions can normally be readily removed from the dehydrogenation effluent stream by a relatively easy fractional distillation step. The diolefinic hydrocarbons which result from dehydrogenation reactions can normally be readily converted to monoolefinic hydrocarbons in the dehydrogenation effluent product stream by a relatively easy catalytic selective hydrogenation step. In contrast, the aromatic hydrocarbons remain in the dehydrogenation effluent stream. The presence of aromatic by-products in an olefinic product stream is often undesirable because the aromatic hydrocarbons are impurities that react in downstream processes to form different compounds than the monoolefinic hydrocarbons.
It is well known that aromatic by-products are formed during the catalytic dehydrogenation of paraffins. For instance, the article starting at page 86 of the Jan. 26, 1970, issue of "Chemical Engineering" states that the product of the dehydrogenation of linear paraffins includes aromatic compounds. The nature of the particular aromatic by-products that are formed in dehydrogenation is not essential to this invention. Without limiting this invention in any way, these aromatic by-products are believed to include, for example, alkylated benzenes, naphthalenes, other polynuclear aromatics, alkylated polynuclear hydrocarbons in the C.sub.10 -C.sub.15 range, indanes and tetralins, that is, they are aromatics of the same carbon number as the paraffin being dehydrogenated and may be viewed as aromatized normal paraffins. The particular side reactions that lead to the formation of the aromatic by-products are also not essential to this invention. Again, without limiting this invention in any way, an illustration of some of the parallel thermal cracking reactions that can lead to the formation of aromatic by-products is found in the diagram at the top of page 4-37 of the book edited by R. A. Meyers entitled "Handbook of Petroleum Refining Processes" (McGraw Hill, N.Y., 1986). Typically, from about 1.0 to about 7.0 weight percent, and generally to the extent of no more than 10.0 weight percent, of the converted feed paraffinic compounds of a dehydrogenation zone form aromatic by-products. Although some commercially available dehydrogenation catalysts are more selective than others at minimizing the formation of aromatic by-products, it is believed that these by-products are formed, at least to a small extent, at suitable dehydrogenation conditions in the presence of most, if not all, commercially-available dehydrogenation catalysts. Since it is an economic advantage to operate the dehydrogenation zone at conditions that produce a high conversion of the feed paraffinic compounds and a high yield of the desired olefins, these aromatic by-products are produced, at least to a small extent, in most if not all commercial dehydrogenation zones.